The photoacoustic imaging system is an apparatus which displays an image based on detection signals of acoustic waves (typically ultrasonic waves) generated by thermal expansion of a measuring object when a living body serving as an examination object is irradiated with light. The diagnostic apparatus examines specific substances in the examination object, e.g., glucose, hemoglobin, and the like contained in blood.
It is known that when a tumor such as a cancer grows in biological tissues, blood vessels around the tumor are newly formed and the oxygen is increasingly consumed by the tumor. As a method of evaluating the formation of new blood vessels and the increase in oxygen consumption, the light absorption coefficient of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) can be utilized. For example, the photoacoustic imaging system measures the concentration of HbO2 and Hb in blood from the light absorption coefficient of HbO2 and Hb, respectively, at a plurality of wavelengths. Then, by creating a concentration distribution image of HbO2 and Hb in the biological tissues, a region where the blood vessels are formed can be identified. Moreover, by calculating the oxygen saturation degree based on the light absorption coefficient ratio of HbO2 and Hb at two or more wavelengths, a region where the oxygen consumption increases, which is considered to be a region where the tumor is present, can be identified. For example, it is known that the oxygen saturation in the tumor region reaches about 70%. Moreover, it is suggested that there is a correlation between the malignancy of the tumor and the oxygen saturation. When identifying the malignancy of the tumor using the oxygen saturation, the accuracy which allows identification of a difference of about 5% in the oxygen saturation is demanded.
Since it is difficult to recognize shape of the region with the photoacoustic imaging system, it is also difficult to distinguish signals based on a tumor from signals resulting from a noise and the like. Therefore, by combining the photoacoustic imaging system with an ultrasonic wave diagnostic apparatus, the identification of a tumor based on shape information can be achieved. To identify a tumor with the ultrasonic wave diagnostic apparatus, it is required to identify an acoustic impedance difference of about 0.1 rayl.
In a phantom for use in the photoacoustic imaging system, a material whose light propagation characteristics and acoustic propagation characteristics are similar to those of human tissues is demanded. For example, PTL 1 discloses a phantom for photoacoustic imaging system employing titanium oxide and polyol in which a carbon black pigment is dispersed.